Process for removing gaseous contaminants from a feed gas stream comprising methane and gaseous contaminants

ABSTRACT

The invention provides a process for removing gaseous contaminants from a feed gas stream which comprises methane and gaseous contaminants, the process comprising: 1) providing the feed gas stream; 2) cooling the feed gas stream to a first temperature at which liquid phase contaminant is formed as well as a methane enriched gaseous phase; 3) separating the two phases obtained in step 2) by means of a first gas/liquid separator; 4) cooling the methane enriched gaseous phase obtained in step 3) at least party by means of an external refrigerant to a second temperature at which liquid phase contaminant is formed as well as a methane enriched gaseous phase; and 5) separating the two phases obtained in step 4) by means of a second gas/liquid separator. The invention further concerns a device for carrying out the present process, the purified gas stream, and a process for liquefying a feed gas stream.

The present invention concerns a process for the removal of gaseouscontaminants from a feed gas stream which comprises methane and gaseouscontaminants, in particular the removal of gaseous contaminants such ascarbon dioxide and hydrogen sulphide from a natural gas.

Methane comprising gas streams produced from subsurface reservoirs,especially natural gas, associated gas and coal bed methane, usuallycontain contaminants as carbon dioxide, hydrogen sulphide, carbonoxysulphide, mercaptans, sulphides and aromatic sulphur containingcompounds in varying amounts. For most of the applications of these gasstreams, the contaminants needs to be removed, either partly or almostcompletely, depending on the specific contaminant and/or the use. Often,the sulphur compounds need to be removed into the ppm level, carbondioxide sometimes into ppm level, e.g. LNG applications, or down to 2 or3 vol. percent, e.g. for use as heating gas. Higher hydrocarbons may bepresent, which, depending on the use, may be recovered.

Processes for the removal of carbon dioxide and sulphur compounds areknow in the art. These processes include absorption processes using e.g.aqueous amine solutions or adsorption processes using e.g. molecularsieves. These processes are especially suitable for the removal ofcontaminants, especially carbon dioxide and hydrogen sulphide, that arepresent in relatively low amounts, e.g. up till several vol %.

In WO 2006/087332, a method has been described for removingcontaminating gaseous components, such as carbon dioxide and hydrogensulphide, from a natural gas stream. In this method a contaminatednatural gas stream is cooled in a first expander to obtain an expandedgas stream having a temperature and pressure at which the dewpointingconditions of the phases containing a preponderance of contaminatingcomponents, such a carbon dioxide and/or hydrogen sulphide are achieved.The expanded gas stream is then supplied to a first centrifugalseparator to establish the separation of a contaminants-enriched liquidphase and a contaminants-depleted gaseous phase. Thecontaminants-depleted gaseous phase is then passed via a recompressor,an interstage cooler, and a second expander into a second centrifugalseparator. The interstage cooler and the second expander are used tocool the contaminants-depleted gaseous phase to such an extent thatagain a contaminants-enriched liquid phase and a furthercontaminants-depleted gaseous phase are obtained which are subsequentlyseparated from each other by means of the second centrifugal separator.In such a method energy recovered from the first expansion step is usedin the compression step, and air, water and/or an internal processstream is used in the interstage cooler.

A disadvantage of this known method is that the use of a recompressor,interstage cooler and an expander between the two centrifugal separatorsaffects the hydrocarbon efficiency of the separation process, whichhydrocarbon efficiency is a measure of the fuel gas consumption and thehydrocarbon loss in the liquid phase contaminant streams during theprocess.

It has now been found that in an integrated process for removing gaseouscontaminants from a gas stream the hydrocarbon efficiency can beconsiderably improved when between a first and a second gas/liquidseparation the contaminants-depleted gaseous phase is at least partlycooled by means of an external refrigerant which allows an excellentseparation of gaseous contaminants, whereas the use of an expanderbetween the first and second gas/liquid separation can be avoided.

Thus, the present invention concerns a process for removing gaseouscontaminants from a feed gas stream which comprises methane and gaseouscontaminants, which process comprises:

1) providing the feed gas stream;2) cooling the feed gas stream to a first temperature at which liquidphase contaminant is formed as well as a methane enriched gaseous phase;3) separating the two phases obtained in step 2) by means of a firstgas/liquid separator;4) cooling the methane enriched gaseous phase obtained in step 3) atleast partly by means of an external refrigerant to a second temperatureat which liquid phase contaminant is formed as well as a methaneenriched gaseous phase; and5) separating the two phases obtained in step 4) by means of a secondgas/liquid separator.

Suitably, the feed gas stream is a natural gas stream in which thegaseous contaminants are carbon dioxide and/or hydrogen sulphide.

The natural gas stream suitably comprises between 1 and 90 vol % ofcarbon dioxide, preferably between 5 and 80 vol % of carbon dioxide.

The natural gas stream suitably comprises between 0.1 to 60 vol % ofhydrogen sulphide, preferably between 20 and 40 vol % of hydrogensulphide.

The feed gas stream to be used in accordance with the present inventioncomprises between 20 and 80 vol % of methane.

Suitably, the feed gas stream in step 1) has a temperature between −20and 150° C., preferably between −10 and 70° C., and a pressure between10 and 150 bara, preferably between 80 and 120 bara.

The raw feed gas stream may be pre-treated to partially or completelyremove water and optionally some heavy hydrocarbons. This can forinstance be done by means of a pre-cooling cycle, against an externalcooling loop or a cold internal process stream. Water may also beremoved by means of pre-treatment with molecular sieves, e.g. zeolites,or silica gel or alumina oxide or other drying agents such as glycol,MEG, DEG or TEG, or glycerol. The amount of water in the gas feed streamis suitably less than 1 vol %, preferably less than 0.1 vol %, morepreferably less than 0.0001 vol %.

The cooling in step 2) of the feed gas stream may be done by methodsknown in the art. For instance, cooling may be done against an externalcooling fluid. In the case that the pressure of the feed gas issufficiently high, cooling may be obtained by expansion of the feed gasstream.

Combinations may also be possible. A suitable method to cool the feedgas stream is done by nearly isentropic expansion, especially by meansof an expander, preferably a turbo expander or laval nozzle. Anothersuitable method is to cool the feed gas stream by isenthalpic expansion,preferably isenthalpic expansion over an orifice or a valve, especiallyover a Joule-Thomson valve.

Preferably, the expansion is done using at least two expansion devicesand the operating parameters of the expansion devices are chosen suchthat the liquefied contaminants in the cooled stream have a certaindroplet size distribution. The use of at least two expansion devicesallows the control of the droplet size distribution of condensedcontaminants.

In a preferred embodiment the feed gas stream is pre-cooled beforeexpansion. This may be done against an external cooling loop or againsta cold process stream, e.g. liquid contaminant. Preferably the gasstream is pre-cooled before expansion to a temperature between 15 and−35° C., preferably between 10° C. and −20° C. Pre-cooling may be doneagainst internal process streams. Especially when the feed gas streamhas been compressed, the temperature of the feed gas stream may bebetween 100 and 150° C. In that case air or water cooling may be used todecrease the temperature first, optionally followed by further cooling.

Another suitable cooling method is heat exchange against a cold fluidum,especially an external refrigerant, e.g. a propane cycle, anethane/propane cascade or a mixed refrigerant cycle, optionally incombination with an internal process loop, suitably a carbon dioxidestream (liquid or slurry), a cold methane enriched stream or washingfluid.

Suitably the feed gas stream is cooled in steps 2) and 4) to atemperature between −30 and −80° C., preferably between −40 and −65° C.At these temperatures liquid phase contaminant will be formed.

Suitably, the pressure applied in step 4) can be higher than thepressure applied in step 2).

Preferably, the second temperature in step 4) is lower than the firsttemperature in step 2).

Preferably, the second temperature in step 4) is up to 20° C. lower thanthe first temperature in step 2). More preferably, the secondtemperature is between 5 and 10° C. lower than the first temperature instep 2).

The cooling in step 4) can be carried out by means of an internalprocess stream such as a stream of liquid phase contaminant which isseparated from the methane enriched gaseous phase in step 3).

In accordance with the present invention the cooling of the methaneenriched gaseous phase in step 4) can suitably at least partly be doneby means of an external refrigerant.

Preferably, the external refrigerant to be used in step 4) has a highermolecular weight than the methane enriched gaseous phase to be cooled.Suitable examples of such cooling medium include ethane, propane andbutane. Preferably, the cooling medium comprises ethane and/or propane.

More preferably, the external refrigerant to be used comprises a propanecycle, an ethane/propane mixed refrigerant or an ethane/propane cascade.Such an ethane/propane cascade is described in more detail hereinbelow.

The cooling in step 4) can suitably be partly done by means of anexternal refrigerant and partly by means of an internal process stream,e.g. a stream of liquid phase contaminant which is separated from themethane enriched gaseous phase in step 3).

The cooling in step 4) as, for instance, done by use of an externalrefrigerant can very attractively replace the sequence of therecompressor, interstage cooler and the expander which is used betweenthe two centrifugal separators as described in WO 2006/087332, improvingthe hydrocarbon efficiency of the separation process.

In another embodiment of the present invention the methane enrichedgaseous phase obtained in step 3) is recompressed in one or morecompression steps before step 4) is carried out.

In another embodiment of the present invention the methane enrichedgaseous phase obtained in step 3) is firstly cooled by means of aninterstage cooler before the cooling in step 4) is carried out.

In yet another embodiment of the present invention, the methane enrichedgaseous phase obtained in step 3) is firstly recompressed in one or morecompression steps, than cooled by means of an interstage cooler, andsubsequently cooled in step 4).

Suitably, such an interstage cooler will be based on a internal processstream and air or water cooling.

Suitably, in the case liquid is formed inside the cooler, this cooler isdesigned in such a way that liquid is effectively removed form thecooling device without impairing heat transfer.

In the one or more compression steps suitably energy is used that isrecovered in step 2).

In the process according to the present invention a variety ofgas/liquid separators can suitably be used in steps 3) and 5), such as,for instance, rotating centrifuges or cyclones.

In steps 3) and 5) use can be made of different types or similar typesof gas/liquid separators. Suitably, in steps 3) and 5) is made ofsimilar types of gas/liquid separators.

Suitable gas/liquid separators to be used in accordance with the presentinvention have, for instance, been described in WO 2008/082291, WO2006/087332, WO 2005/118110, WO 97/44117, WO 2007/097621 and WO94/23823, which documents are hereby incorporated by reference.

Typically, the gas/liquid separator requirements in step 3) are morestringent than the requirements in step 5) since homogeneous dropletnucleation after expansion does produce smaller droplets thanheterogeneous nucleation in a heat exchanger, cooled by an externalprocess stream.

In a preferred embodiment of the present invention, the first and/orsecond gas/liquid separator comprises a gas/liquid inlet at anintermediate level, a liquid outlet arranged below the gas/liquid inletand a gas outlet arranged above the gas/liquid inlet, in which vessel anormally horizontal coalescer is present above the gas/liquid inlet andover the whole cross-section of the vessel and in which vessel acentrifugal liquid separator is arranged above the coalescer and overthe whole cross-section of the vessel, the liquid separator comprisingone or more swirl tubes.

When using a vertical gas/liquid separator vessel, the process onlyneeds a relatively small area.

According to a preferred embodiment, the gas/liquid inlet comprises anadmittance with a supply and distribution assembly extendinghorizontally in the separator vessel. In its most simple form, the inletis a simple pipe, having a closed end and a number of perforationsevenly distributed over the length of the pipe. Optionally, the pipe mayhave a tapered or conical shape. One or more cross pipes may be presentto create a grid system to distribute the gas-liquid mixture more evenlyover the cross-section of the vessel. Preferably, the assembly includesa chamber, e.g. a longitudinal box-like structure, connected to the gasinlet and having at least one open vertical side with a grid of guidevanes disposed one behind each other, seen in the direction of the flow.By means of this supply and distribution assembly, the gas is evenlydistributed by the guide vanes over the cross-section of the column,which brings about an additional improvement of the liquid separation inthe coalescer/centrifugal separator combination. A further advantage isthat the supply and distribution assembly separates from the gas anyslugs of liquid which may suddenly occur in the gas stream, theseparation being effected by the liquid colliding with the guide vanesand falling down inside the column. Suitably, the box structure narrowsdown in the direction of the flow. After having been distributed by thevanes over the column cross-section, the gas flows up to the coalescer.

In a preferred embodiment the longitudinal chamber has two open verticalsides with a grid of guide vanes.

Suitable gas/liquid inlets are those described in e.g. GB 1,119,699,U.S. Pat. No. 6,942,720, EP 195,464, U.S. Pat. No. 6,386,520 and U.S.Pat. No. 6,537,458. A suitable, commercially available gas/liquid inletis a Schoepentoeter.

There are numerous horizontal coalescers available, especially forvertical columns. A well-known example of a mist eliminator is thedemister mat. All of these are relatively tenuous (large permeability)and have a relatively large specific (internal) surface area. Theiroperation is based on drop capture by collision of drops with internalsurfaces, followed by drop growth on these surfaces, and finally byremoval of the grown drop either by the gas or by gravity.

The horizontal coalescer can have many forms which are known per se andmay, for example, consist of a bed of layers of gauze, especially metalor non-metal gauze, e.g. organic polymer gauze, or a layer of vanes or alayer of structured packing. Also unstructured packings can be used andalso one or more trays may be present. All these sorts of coalescershave the advantage of being commercially available and operatingefficiently in the column according to the invention. See also Perry'sChemical Engineers' Handbook, Sixth edition, especially Chapter 18. Seealso EP 195464.

The centrifugal liquid separator in one of its most simple forms maycomprise a horizontal plate and one or more vertical swirl tubesextending downwardly from the plate, each swirl tube having one or moreliquid outlets below the horizontal plate at the upper end of the swirltube. In another form, the centrifugal liquid separator comprises one ormore vertical swirl tubes extending upwardly from the plate, each swirltube having one or more liquid outlets at the upper end. The plate isprovided with a downcomer, preferably a downcomer that extends to thelower end of the separator vessel.

In a preferred embodiment of the invention, the centrifugal liquidseparator comprises two horizontal trays between which verticalopen-ended swirl tubes extend, each from an opening in the lower tray tosome distance below a coaxial opening in the upper tray, means for thedischarge of secondary gas and of liquid from the space between thetrays outside the swirl tubes, and means provided in the lower part ofthe swirl tubes to impart to the gas/liquid a rotary movement around thevertical axis.

The liquid separator is also preferably provided with vertical tubepieces which project down from the coaxial openings in the upper trayinto the swirl tubes and have a smaller diameter than these latter. Thisarrangement enhances the separation between primary gas on the one handand secondary gas and liquid on the other hand, since these lattercannot get from the swirl tubes into the openings in the upper tray forprimary gas.

According to a preferred embodiment, the means for discharging thesecondary gas from the space between the trays consist of verticaltubelets through the upper tray, and the means for discharging liquidfrom the space between the trays consist of one or more verticaldischarge pipes which extend from this space to the bottom of thecolumn. This arrangement has the advantage that the secondary gas, afterhaving been separated from liquid in the said space between the trays,is immediately returned to the primary gas, and the liquid is added tothe liquid at the bottom of the column after coming from the coalescer,so that the secondary gas and the liquid removed in the centrifugalseparator do not require separate treatment.

In order to improve even further the liquid separation in thecentrifugal separator, openings are preferably provided in accordancewith the invention at the top of the swirl tubes for discharging liquidto the space between the trays outside the swirl tubes. This has theadvantage that less secondary gas is carried to the space between thetrays. A suitable, commercially available centrifugal separator is aShell Swirltube deck.

In a preferred embodiment, the separation vessel comprises a secondnormally horizontal liquid coalescer above the centrifugal liquidseparator and over the whole cross-section of the vessel. This has theadvantage that any droplets still present in the gas stream are removed.See for a further description hereinabove. Preferably, the secondcoalescer is a bed of one or more layers of gauze, especially metal ornon-metal gauze, e.g. organic polymer gauze. In another preferredembodiment, the second normally horizontal liquid coalescer is situatedabove the secondary gas outlets, for instance in the way as described inEP 83811, especially as depicted in FIG. 4.

In a most preferred embodiment, a first and/or second separator is usedcomprising:

a) a housing comprising a first, second and third separation section forseparating liquid from the mixture, wherein the second separationsection is arranged below the first separation section and above thethird separation section, the respective separation sections are incommunication with each other, and the second separation sectioncomprises a rotating coalescer element;b) tangentially arranged inlet means to introduce the mixture into thefirst separation section;c) means to remove liquid from the first separation section;d) means to remove liquid from the third separation section; ande) means to remove a gaseous stream, lean in liquid, from the thirdseparation section.

In the present invention the first and/or second gas/liquid separatormay suitably comprise a centrifugal separator which comprises a bundleof parallel channels that are arranged within a spinning tube parallelto an axis of rotation of the spinning tube.

Suitably, in the present process the centrifugal separator is spinned byintroducing a swirling gas stream into the spinning tube.

Preferably, the centrifugal separator to be used in accordance with thepresent invention comprises a housing with a gas inlet for contaminatedgas at one end of the vessel, a separating body, a gas outlet forpurified gas at the opposite end of the housing and a contaminantsoutlet downstream of the separating body or upstream and downstream ofthe separating body, wherein the separating body comprises a pluralityof ducts over a part of the length of the axis of the housing, whichducts have been arranged around a central axis of rotation, in whichapparatus the separating body has been composed of a plurality ofperforated discs wherein the perforations of the discs form the ducts.

It will be appreciated that the discs can be easily created by drillingor cutting a plurality of perforations into the relatively thin discs.By attaching several discs to together these discs form a separatingbody. By aligning the perforations ducts are obtained.

It is now also very easy to attach the discs such that the perforationsare not completely aligned. By varying the number and nature of thenon-alignment of the perforations the resulting ducts can be given anydesired shape. In such cases not only ducts are obtainable that are notcompletely parallel to the central axis of rotation, but also ducts thatform a helix shape around the axis of rotation. So, in this way veryeasily the preferred embodiment of having non-parallel ducts can beobtained. Hence it is preferred that the perforations of the discs havebeen arranged such that the ducts are not parallel to the central axisof rotation or form a helix shape around the axis of rotation.

Further, it will be appreciated that it is relatively easy to increaseor decrease the diameter of the perforations. Thereby the skilled personhas an easy manner at his disposal to adapt the (hydraulic) diameter ofthe ducts, and thereby the Reynolds number, so that he can easyascertain that the flow in the ducts is laminar or turbulent, just as hepleases. The use of these discs also enables the skilled person to varythe diameter of the duct along the axis of the housing. The varyingdiameter can be selected such that the separated liquid or solidcontaminants that are collected against the wall of the duct will notclog up the duct completely, which would hamper the operation of theapparatus.

The skilled person is also now enabled to maximise the porosity of theseparating body. The easy construction of the discs allows the skilledperson to meticulously provide the disc with as many perforations as helikes. He may also select the shape of the perforations. These may havea circular cross-section, but also square, pentagon, hexagon, octagon oroval cross-sections are possible. He may therefore minimise the wallthickness of the separating body and the wall thicknesses of the ducts.He is able to select the wall thicknesses and the shape of the ductssuch that the surface area that is contributed to the cross-section ofthe separating body by the walls is minimal. That means that thepressure drop over the separating body can be minimised.

The apparatus can have a small or large number of ducts. Just asexplained in the prior art apparatuses the number of ducts suitablyranges from 100 to 1,000,000, preferably from 500 to 500,000. Thediameter of the cross-section of the ducts can be varied in accordancewith the amount of gas and amounts and nature, e.g., droplet sizedistribution, of contaminants and the desired contaminants removalefficiency. Suitably, the diameter is from 0.05 to 50 mm, preferablyfrom 0.1 to 20 mm, and more preferably from 0.1 to 5 mm. By diameter isunderstood twice the radius in case of circular cross-sections or thelargest diagonal in case of any other shape.

The size of the apparatus and in particular of the separating body mayvary in accordance with the amount of gas to be treated. In EP-B 286 160it is indicated that separating bodies with a peripheral diameter of 1 mand an axial length of 1.5 m are feasible. The separating body accordingto the present invention may suitably have a radial length ranging from0.1 to 5 m, preferably from 0.2 to 2 m. The axial length rangesconveniently from 0.1 to 10 m, preferably, from 0.2 to 5 m.

The number of discs may also vary over a large number. It is possible tohave only two discs if a simple separation is needed and/or when theperforations can be easily made. Other considerations may be whetherparallel ducts are desired, or whether a uniform diameter is wanted.Suitably the number of discs varies from 3 to 1000, preferably from 4 to500, more preferably from 4 to 40. When more discs, are used the skilledperson will find it easier to gradually vary the diameter of the ductsand/or to construct non-parallel ducts. Moreover, by increasing ordecreasing the number of discs the skilled person may vary the ductlength. So, when the conditions or the composition of the gas changes,the skilled person may adapt the duct length easily to provide the mostoptimal conditions for the apparatus of the present invention. The sizeof the discs is selected such that the radial diameter suitably rangesfrom 0.1 to 5 m, preferably from 0.2 to 2 m. The axial length of thediscs may be varied in accordance with construction possibilities,desire for varying the shape etc. Suitably, the axial length of eachdisc ranges from 0.001 to 0.5 m, preferably from 0.002 to 0.2 m, morepreferably from 0.005 to 0.1 m.

Although the discs may be manufactured from a variety of materials,including paper, cardboard, and foil, it is preferred to manufacture thediscs from metal or ceramics. Metals discs have the advantage that theycan be easily perforated and be combined to firm sturdy separatingbodies. Dependent on the material that needs to be purified a suitablemetal can be selected. For some applications carbon steel is suitablewhereas for other applications, in particular when corrosive materialsare to be separated, stainless steel may be preferred. Ceramics have theadvantage that they can be extruded into the desired form such as inhoneycomb structures with protruding ducts.

Typically, the ceramics precursor material is chosen to form a dense orlow-porosity ceramic. Thereby the solid or liquid contaminants areforced to flow along the wall of the ducts and not, or hardly, throughthe ceramic material of the walls. Examples of ceramic materials aresilica, alumina, zirconia, optionally with different types andconcentrations of modifiers to adapt its physical and/or chemicalproperties to the gas and the contaminants.

The discs may be combined to a separating body in a variety of ways. Theskilled person will appreciate that such may depend on the material fromwhich the discs have been manufactured. A convenient manner is to attachthe discs to a shaft that provides the axis of rotation. Suitable waysof combining the discs include clamping the discs together, but alsogluing them or welding them together can be done. Alternatively, thediscs may be stacked in a cylindrical sleeve. This sleeve may also atleast partly replace the shaft. This could be convenient for extrudeddiscs since no central opening for the shaft would be required. It ispreferred to have metal discs that are welded together.

In a preferred embodiment of the invention, the methane enriched gaseousphase obtained in accordance with the present invention is furtherpurified, e.g. by extraction of remaining acidic components with achemical solvent, e.g. an aqueous amine solution, especially aqueousethanolamines, such as DIPA, DMA, MDEA, etc., or with a physicalsolvent, e.g. cold methanol, DEPG, NMP, etc.

The contaminated gas stream is continuously provided, continuouslycooled and continuously separated.

The present invention also relates to a device (plant) for carrying outthe process as described above, as well as the purified gas streamobtained by the present process. In addition, the present inventionconcerns a process for liquefying a feed gas stream comprising purifyingthe feed gas stream by means of the present process, followed byliquifying the purified feed gas stream by methods known in the art.

The invention will be further illustrated by means of the followingFigures.

Referring to FIG. 1, natural gas via a conduit 1 is passed through anexpansion means 2, whereby a stream is obtained comprising liquid phasecontaminant and a methane enriched gaseous phase. The stream flows via aconduit 3 into a gas/liquid separator 4 wherein the two phases areseparated from each other. The liquid phase contaminant is recovered viaa conduit 5, whereas the methane enriched gaseous phase is passed via aconduit 6 into a heat exchanger 7. In heat exchanger 7 ethane is used asan external refrigerant whereby the ethane is cooled by means of anethane/propane cascade 8 as depicted in more detail in FIG. 2. Themethane enriched gaseous phase is cooled in the heat exchanger 7 to atemperature whereby a liquid phase contaminant and an methane enrichedgaseous phase are formed. The stream which comprises these two phases isthen passed via a conduit 9 into a gas/liquid separator 10 from which afurther enriched methane enriched gaseous phase is recovered via aconduit 11 and liquid phase contaminant is recovered via a conduit 12.

In FIG. 2 the heat exchanger 7 is shown using ethane that is cooled bymeans of an ethane/propane cascade which comprises an ethane loop and apropane loop. In the ethane loop an ethane stream is passed via aconduit 13 into an expander 14 (e.g. a turbine expander or aJoule-Thomson valve), and the cooled ethane stream so obtained is passedvia a conduit 15 into the heat exchanger 7. A stream of warm ethane isthen passed from the heat exchanger 7 to a recompressor 16 via a conduit17 to increase the pressure of the ethane stream. The compressed streamof ethane obtained from recompressor 16 is then passed via a conduit 18into a heat exchanger 19 wherein the ethane stream is cooled and atleast partly condensed. Via the conduit 13 the ethane stream is thenrecycled to the expander 14. In the propane loop a propane stream ispassed via a conduit 20 into an expander 21 (e.g. a turbine expander ora Joule-Thomson valve), and the cooled propane stream so obtained ispassed via a conduit 22 into the heat exchanger 19 of the ethane loop. Astream of warm propane is then passed from the heat exchanger 19 via aconduit 23 into a recompressor 24 to increase the pressure of thepropane stream. The compressed stream of propane obtained fromrecompressor 24 is then passed via a conduit 25 into heat exchanger 26wherein the propane stream is cooled and at least partly condensed bymeans of water or air. Via the conduit 20 the propane stream is thenrecycled to the expander 21.

In FIG. 3 a suitable gas/liquid separator is shown for use in steps 3)and 5) of the present process. Both the gas/liquid separators 4 and 10as shown in FIG. 1 can be of this type. The stream comprising liquidphase contaminant and a methane enriched gaseous phase is passed via theconduit 3 (or the conduit 9) into the gas/liquid separator 4 (or thegas/liquid separator 10) via supply and distribution assembly 27. Mostof the liquid will flow down to the lower end of the separator and leavethe separator via the liquid outlet 5. The gaseous stream comprisinglarger and smaller droplets will flow upwards via liquid coalescer 28,centrifugal separator 29 and a second liquid coalescer 30 to the top ofthe separator vessel, and leave the separator vessel via the gas outlet6.

In FIG. 4 another suitable gas/liquid separator is shown for use insteps 3) and 5) of the present process. Both the gas/liquid separators 4and 10 as shown in FIG. 1 can be of this type. The stream comprisingliquid phase contaminant and a methane enriched gaseous phase is passedvia the conduit 3 (or the conduit 9) to a gas inlet 31 in a housing 32of the gas/liquid separator 4 (or the gas/liquid separator 10). Thehousing 32 further comprises a separating body 33 which shows a largenumber of ducts 34 which are arranged around a shaft 35, which providesan axis of rotation. Separating body 33 has been composed of six discs33 a, 33 b, 33 c, 33 d, 33 e and 33 f that have been combined by weldingor gluing. In the rotating separating body droplets of carbon dioxideand/or hydrogen sulphide are separated from the natural gas. Theseparated contaminants are discharged from the housing via acontaminants outlet 36 which has been arranged downstream of theseparating body 33, and via the discharge conduit 5. Purified naturalgas leaves housing 32 via the gas outlet 6 arranged at the opposite endof the housing 32.

1. A process for removing gaseous contaminants from a feed gas streamwhich comprises methane and gaseous contaminants, the processcomprising: 1) providing the feed gas stream; 2) cooling the feed gasstream to a first temperature at which liquid phase contaminant isformed as well as a methane enriched gaseous phase; 3) separating thetwo phases obtained in step 2) by means of a first gas/liquid separator;4) cooling the methane enriched gaseous phase obtained in step 3) atleast partly by means of an external refrigerant to a second temperatureat which liquid phase contaminant is formed as well as a methaneenriched gaseous phase; and 5) separating the two phases obtained instep 4) by means of a second gas/liquid separator.
 2. A processaccording to claim 1, in which at least one of the first and secondgas/liquid separators comprises a centrifugal which comprises a bundleof parallel channels that are arranged within a spinning tube parallelto an axis of rotation of the spinning tube.
 3. A process according toclaim 1, in which the first and/or second gas/liquid separator comprisesa housing with a gas inlet for contaminated gas at one end of thevessel, a separating body, a gas outlet for purified gas at the oppositeend of the housing and a contaminants outlet downstream of theseparating body wherein the separating body comprises a plurality ofducts over a part of the length of the axis of the housing, which ductshave been arranged around a central axis of rotation, in which apparatusthe separating body has been composed of a plurality of perforated discswherein the perforations of the discs form the ducts.
 4. A gasseparation system, comprising: a) a housing comprising a first, secondand third separation section for separating liquid from a gas mixture,wherein the second separation section is arranged below the firstseparation section and above the third separation section, therespective separation sections are in communication with each other, andthe second separation section comprises a rotating coalescer element; b)tangentially arranged inlet means to introduce the mixture into thefirst separation section; c) means to remove liquid from the firstseparation section; d) means to remove liquid from the third separationsection; and e) means to remove a gaseous stream, lean in liquid, fromthe third separation section.
 5. A process according claim 1, in whichthe feed gas stream is a natural gas stream in which the gaseouscontaminants are carbon dioxide and/or hydrogen sulphide.
 6. A processaccording to claim 5, in which the natural gas stream comprises between1 and 90 vol % of carbon dioxide, and between 0.1 and 60 vol % ofhydrogen sulphide.
 7. A process claim 1, in which the feed gas streamcomprises between 20 and 80 vol % of methane.
 8. A process according toclaim 1, in which the feed gas stream in step 1) has a temperaturebetween −20 and 150° C., preferably between −10 and 70° C., and apressure between 10 and 150 bara, preferably between 80 and 120 bara. 9.A process according to claim 1, in which the cooling in step 2) is doneby isenthalpic expansion.
 10. A process according to claim 9, whereinthe expansion is done using at least two expansion devices and theoperating parameters of the expansion devices are chosen such that theliquefied acidic contaminants have a certain droplet size distribution.11. A process claim 1, in which the feed gas stream is cooled in steps2) and 4) to a temperature between −30 and −80° C., preferably between−40 and −65° C.
 12. A process claim 1, in which the pressure applied instep 4) is higher than the pressure applied in step 2), and in which thesecond temperature in step 4) is lower than the first temperature instep 2).
 13. A process according claim 1, in which the externalrefrigerant has a higher molecular weight than the methane enrichedgaseous phase to be cooled.
 14. A process according to claim 1, in whichthe first and/or second gas/liquid separator comprises a gas/liquidinlet at an intermediate level, a liquid outlet arranged below thegas/liquid inlet and a gas outlet arranged above the gas/liquid inlet,in which vessel a normally horizontal coalescer is present above thegas/liquid inlet and over the whole cross-section of the vessel and inwhich vessel a centrifugal liquid separator is arranged above thecoalescer and over the whole cross-section of the vessel, the liquidseparator comprising one or more swirl tubes.
 15. (canceled)